1. Technical Field of the Invention
The present invention relates to communication systems and, more particularly, to power amplifiers used within transmitters.
2. Description of Related Art
Modern wireless RF transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes, such as frequency shift keying (FSK), phase shift keying (PSK), and variants thereof, often in combination with code division multiple access (CDMA) communication. Independent of the particular communications scheme employed, the RF transmitter output signal, sRF(t), can be represented mathematically assRF(t)=r(t)cos(2πfct+θ(t))  (1)where fc denotes the RF carrier frequency, and the signal components r(t) and θ(t) are referred to as the envelope (amplitude) and phase of sRF(t), respectively.
Some of the above mentioned communication schemes have constant envelope, i,e.,r(t)=R, and these are thus referred to as constant-envelope communications schemes. In these communications schemes, θ(t) constitutes all of the information bearing part of the transmitted signal. Other communications schemes have envelopes that vary with time and these are thus referred to as variable-envelope communications schemes. In these communications schemes, both r(t) and θ(t) constitute information bearing parts of the transmitted signal.
A transmitter appropriate for a variable-envelope modulation scheme is a polar transmitter. In a polar transmitter, digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to some intermediate frequency (IF) carrier fIF to generate digital envelope (amplitude-modulated) and digital phase-modulated signals. The digital amplitude-modulated signal is input to a digital-to-analog converter (DAC), followed by a low pass filter (LPF), along an amplitude path, and the digital phase-modulated signal is input to another DAC, followed by another LPF, along a phase path. The output of the LPF on the amplitude path is an analog amplitude signal, while the output of the LPF on the phase path is an analog reference signal. The analog reference signal is input to a phase locked loop to enable the phase of the RF output signal to track the phase of the analog reference signal. The RF output signal is modulated in a non-linear power amplifier (PA) by the analog amplitude-modulated signal.
Thus, in polar transmitter architectures, the phase component of the RF signal is amplified through the non-linear PA while the amplitude modulation is performed at the output of the PA. This architecture, however, requires phase and amplitude alignment to make sure that the amplitude modulated and phase modulated data are applied at the right instant.
In addition, polar transmitters also have several challenges related to amplitude modulation and power control. Conventional amplitude modulation techniques are typically based on the modulation of the power supply. However, the amplitude component of the RF signal occupies several times more bandwidth than the combination of the phase and amplitude data. Therefore, conventional power supply modulation techniques are limited for many wideband applications. In addition, in many wireless systems, the output power must be controlled in order to keep the received signal from reaching all users at the same power level. However, in switching power amplifiers, the power control is performed using the same method as that used for amplitude modulation. As a result, in switching power amplifiers, there is a trade off between the power control dynamic range and the resolution of the amplitude modulation.
Therefore, a need exists for a switching power amplifier that provides high bandwidth, high resolution amplitude modulation capability as well as high power control dynamic range.